The invention relates to a method of manufacturing an optical fiber, whereby a fiber is drawn from a molten extremity of a preform and is subsequently subjected to a torque, thereby causing a portion of the fiber to be twisted about its longitudinal axis and to be endowed with a spin.
The invention also relates to an optical fiber comprising a core portion which is surrounded by an optical cladding portion, itself surrounded by a protective coating. In particular, the invention relates to an optical fiber of this type having low polarisation mode dispersion.
The term Polarisation Mode Dispersion (PMD) refers to the dispersion of a signal propagating through an optical fiber (particularly a single mode fiber), as a result of birefringence in the fiber's core portion. This birefringence is generally caused by imperfections in the fiber, such as slight non-circularity of its core cross-section, asymmetrical lateral stress, etc., and manifests itself in dissimilar effective refractive indices for the carried signal's two orthogonal polarisation modes. In the case of a perfect fiber devoid of PMD, these two modes propagate independently of one another at a common velocity. However, in the presence of PMD, a sizeable phase difference can accumulate between the two modes.
A known method of combatting PMD is to deliberately twist the warm fiber as it is drawn from the preform, so that a mechanical spin becomes "frozen" into the fiber as it cools. The resulting stress in the fiber produces continual mode-coupling between the orthogonal polarisation modes of a carried signal, thereby inhibiting the accumulation of a significant phase lag between the two modes, and consequently causing a significant reduction in the fiber's PMD.
A method as specified in the opening paragraph is known from United States Patent U.S. Pat. No. 5,298,047, wherein the drawn fiber is caused to pass over a pulley whose rotational axis can be canted, so that the pulley can be caused to rock back and forth about an axis perpendicular to its rotational axis. The pulley has a cylindrical surface which is bounded along both circular edges by a protrusive retaining flange, the moving fiber passing between these two flanges. The rocking motion of the pulley produces a twist in the fiber along a substantial portion of its length. In particular, portions of warm fiber which are twisted in this manner will become endowed with a permanent twist (spin) as their constituent material subsequently cools.
The cited document stipulates that the spin imparted to the fiber ideally has a non-constant spatial frequency. This can be achieved by canting the pulley back and forth in a non-periodic manner. In this way, the described method aims to achieve a PMD of less than 0.5 ps/km.sup.1/2.
The known method has a number of drawbacks. For example, despite the presence of the retaining flanges, the moving fiber can easily slip off the canting pulley, especially at high drawing speeds. For this reason, both the canting angle .theta. and the canting frequency must be kept relatively small, which in turn substantially limits the extent to which the fiber can be twisted. Moreover, the canting motion of the pulley causes excessive vibration of the moving fiber, leading to coating inaccuracies, for example. Such problems can generally only be alleviated with the aid of additional stabilisation pulleys. In addition, the requirement that the pulley be canted back and forth in a non-periodic manner generally necessitates relatively non-straightforward actuation means, such as an actuator which is governed by an electronic random number generator, for example.